Composition Control For Photovoltaic Thin Film Manufacturing

ABSTRACT

The present invention relates to methods and apparatus for providing composition control to thin compound semiconductor films for radiation detector and photovoltaic applications. In one aspect of the invention, there is provided a method in which the molar ratio of the elements in a plurality of layers are detected so that tuning of the multi-element layer can occur to obtain the multi-element layer that has a predetermined molar ratio range. In another aspect of the invention, there is provided a method in which the thickness of a sub-layer and layers thereover of Cu, In and/or Ga are detected and tuned in order to provide tuned thicknesses that are substantially the same as pre-determined thicknesses.

CLAIM OF PRIORITY

This application claims priority to and incorporates by reference hereinU.S. Provisional Appln. Ser. No. 60/744,252 filed Apr. 4, 2006 entitled“Composition Control For Photovoltaic Thin Film Manufacturing”.

FIELD OF THE INVENTION

The present invention relates to methods and apparatus for preparingthin films of compound semiconductor films for radiation detector andphotovoltaic applications.

BACKGROUND

Solar cells are photovoltaic devices that convert sunlight directly intoelectrical power. The most common solar cell material is silicon, whichis in the form of single or polycrystalline wafers. However, the cost ofelectricity generated using silicon-based solar cells is higher than thecost of electricity generated by the more traditional methods.Therefore, since early 1970's there has been an effort to reduce cost ofsolar cells for terrestrial use. One way of reducing the cost of solarcells is to develop low-cost thin film growth techniques that candeposit solar-cell-quality absorber materials on large area substratesand to fabricate these devices using high-throughput, low-cost methods.

Group IBIIIAVIA compound semiconductors comprising some of the Group IB(Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se,Te, Po) materials or elements of the periodic table are excellentabsorber materials for thin film solar cell structures. Especially,compounds of Cu, In, Ga, Se and S which are generally referred to asCIGS(S), or Cu(In,Ga)(S,Se)₂ or CuIn_(l-x)Ga_(x) (S_(y)Se_(1-y))_(k),where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employedin solar cell structures that yielded conversion efficienciesapproaching 20%. Absorbers containing Group IIIA element Al and/or GroupVIA element Te also showed promise. Therefore, in summary, compoundscontaining: i) Cu from Group IB, ii) at least one of In, Ga, and Al fromGroup IIIA, and iii) at least one of S, Se, and Te from Group VIA, areof great interest for solar cell applications.

The structure of a conventional Group IBIIIAVIA compound photovoltaiccell such as a Cu(In,Ga,Al)(S,Se,Te)₂ thin film solar cell is shown inFIG. 1A. The device 10 is fabricated on a substrate 11, such as a sheetof glass, a sheet of metal, an insulating foil or web, or a conductivefoil or web. The absorber film 12, which comprises a material in thefamily of Cu(In,Ga,Al)(S,Se,Te)₂, is grown over a conductive layer 13 ora contact layer, which is previously deposited on the substrate 11 andwhich acts as the electrical contact to the device. Various conductivelayers comprising Mo, Ta, W, Ti, and stainless steel etc. have been usedin the solar cell structure of FIG. 1. If the substrate itself is aproperly selected conductive material, it is possible not to use aconductive layer 13, since the substrate 11 may then be used as theohmic contact to the device. After the absorber film 12 is grown, atransparent layer 14 such as a CdS, ZnO or CdS/ZnO stack is formed onthe absorber film. Radiation 15 enters the device through thetransparent layer 14. Metallic grids (not shown) may also be depositedover the transparent layer 14 to reduce the effective series resistanceof the device. It should be noted that the structure of FIG. 1A may alsobe inverted if substrate is transparent. In that case light enters thedevice from the substrate side of the solar cell.

In a thin film solar cell employing a Group IBIIIAVIA compound absorber,the cell efficiency is a strong function of the molar ratio of IB/IIIA.If there are more than one Group IIIA materials in the composition, therelative amounts or molar ratios of these IIIA elements also affect theproperties. For a Cu(In,Ga)(S,Se)₂ absorber layer, for example, theefficiency of the device is a function of the molar ratio of Cu/(In+Ga).Furthermore, some of the important parameters of the cell, such as itsopen circuit voltage, short circuit current and fill factor vary withthe molar ratio of the IIIA elements, i.e. the Ga/(Ga+In) molar ratio.In general, for good device performance Cu/(In+Ga) molar ratio is keptat around or below 1.0. As the Ga/(Ga+In) molar ratio increases, on theother hand, the optical bandgap of the absorber layer increases andtherefore the open circuit voltage of the solar cell increases while theshort circuit current typically may decrease. It is important for a thinfilm deposition process to have the capability of controlling both themolar ratio of IB/IIIA, and the molar ratios of the Group IIIAcomponents in the composition. It should be noted that although thechemical formula is often written as Cu(In,Ga)(S,Se)₂, a more accurateformula for the compound is Cu(In,Ga)(S,Se)_(k), where k is typicallyclose to 2 but may not be exactly 2. For simplicity we will continue touse the value of k as 2. It should be further noted that the notation“Cu(X,Y)” in the chemical formula means all chemical compositions of Xand Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example,Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly,Cu(In,Ga)(S,Se)₂ means the whole family of compounds with Ga/(Ga+In)molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from0 to 1.

The first technique used to grow Cu(In,Ga)Se₂ layers was theco-evaporation approach which involves evaporation of Cu, In, Ga and Sefrom separate evaporation boats onto a heated substrate, as thedeposition rate of each component is carefully monitored and controlled.

Another technique for growing Cu(In,Ga)(S,Se)₂ type compound thin filmsfor solar cell applications is a two-stage process where at least two ofthe components of the Cu(In,Ga)(S,Se)₂ material are first deposited ontoa substrate, and then reacted with S and/or Se in a high temperatureannealing process. For example, for CuInSe₂ growth, thin sub-layers ofCu and In are first deposited on a substrate to form a precursor layerand then this stacked precursor layer is reacted with Se at elevatedtemperature. If the reaction atmosphere contains sulfur, then aCuIn(S,Se)₂ layer can be grown. Addition of Ga in the precursor layer,i.e. use of a Cu/In/Ga stacked film precursor, allows the growth of aCu(In,Ga)(S,Se)₂ absorber. Other prior-art techniques include depositionof Cu—Se/In—Se, Cu—Se/Ga—Se, or Cu—Se/In—Se/Ga—Se stacks and theirreaction to form the compound. Mixed precursor stacks comprisingcompound and elemental sub-layers, such as a Cu/In—Se stack or aCu/In—Se/Ga—Se stack, have also been used, where In—Se and Ga—Serepresent selenides of In and Ga, respectively.

Sputtering and evaporation techniques have been used in prior artapproaches to deposit the sub-layers containing the Group IB and GroupIIIA components of metallic precursor stacks. In the case of CuInSe₂growth, for example, Cu and In sub-layers were sequentiallysputter-deposited from Cu and In targets on a substrate and then thestacked precursor film thus obtained was heated in the presence of gascontaining Se at elevated temperatures as described in U.S. Pat. No.4,798,660. More recently U.S. Pat. No. 6,048,442 disclosed a methodcomprising sputter-depositing a stacked precursor film comprising aCu—Ga alloy sub-layer and an In sub-layer to form a Cu—Ga/In stack on ametallic back electrode and then reacting this precursor stack film withone of Se and S to form the compound absorber layer. U.S. Pat. No.6,092,669 described sputtering-based equipment and method for producingsuch absorber layers.

One prior art method described in U.S. Pat. No. 4,581,108 utilizes a lowcost electrodeposition approach for metallic precursor preparation. Inthis method a Cu sub-layer is first electrodeposited on a substrate.This is then followed by electrodeposition of an In sub-layer andheating of the deposited Cu/In precursor stack in a reactive atmospherecontaining Se. Methods of fabricating more advanced metallic precursorstacks comprising Cu, In and Ga have recently been described (SP-001,SP-003, SP-004, SP-006, SP007 applications).

Irrespective of the specific approach used to grow a Cu(In,Ga)(S,Se)₂absorber film, two molar ratios mentioned before, i.e. the Cu/(In+Ga)ratio and the Ga/(Ga+In) ratio, should be closely controlled from run torun and on large area substrates. In co-evaporation techniques, thiscomposition control is achieved through in-situ monitoring of theevaporation rates of Cu, In, Ga and Se. In two-stage techniques, whichinvolve deposition of sub-layers to form a precursor film and thenreaction of the precursor film to form the compound absorber layer,individual thicknesses of the sub-layers forming the stacked precursorfilm layer need to be well controlled because they determine the finalstoichiometry or composition of the compound layer after the reactionstep. If the sub-layers are deposited by vacuum approaches, such asevaporation and sputtering, thicknesses of the sub-layers within theprecursor layer (such as thicknesses of Cu and In sub-layers) may bemonitored and controlled using in-situ measurement devices such ascrystal oscillators, or sensors that sense the material deposition flux.In the prior art electroplating techniques, on the other hand, thicknessof individual sub-layers such as the Cu sub-layer, In sub-layer, and/orGa sub-layer have been controlled through control of the charge passedduring deposition of the sub-layer, i.e. through control of thedeposition current density and the deposition time. However, duringprocessing, it is possible that deposition rates change. For example, inelectrodeposition approach the deposition rates may vary due to reasonssuch as a change in the electrodeposition efficiency, age of thedeposition bath, accumulation of byproducts in the bath, change inorganic or inorganic additive concentrations and/or activities etc. inthe electrolyte. Since solar cell efficiency is a strong function of themolar ratios of the elements within the deposited precursors, newtechniques that assure excellent control of these ratios are needed.

SUMMARY OF THE INVENTION

The present invention relates to methods and apparatus for providingcomposition control to thin films of compound semiconductor films forradiation detector and photovoltaic applications.

In one aspect of the invention, there is provided a method in which themolar ratio of the elements in a plurality of layers are detected sothat tuning of the multi-element layer can occur to obtain themulti-element layer that has a predetermined molar ratio range.

In another aspect of the invention, there is provided a method in whichthe thickness of a sub-layer and layers thereover of Cu, In and/or Gaare detected and tuned in order to provide tuned thicknesses that aresubstantially the same as pre-determined thicknesses.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and features of the present invention willbecome apparent to those of ordinary skill in the art upon review of thefollowing description of specific embodiments of the invention inconjunction with the accompanying figures, wherein:

FIG. 1A is a cross-sectional view of a solar cell employing a GroupIBIIIAVIA absorber layer.

FIG. 1B is a representation of the process steps of one embodiment.

FIG. 2A shows a cluster tool configuration with various chambers arounda central handler.

FIG. 2B shows an in-line tool configuration with composition controlcapability in accordance with one embodiment of the invention.

FIG. 3 shows an integrated deposition station/metrology station pair.

FIG. 4 shows a roll-to-roll electrodeposition system employing at leastone deposition station, at least one metrology station and at least onetuning station.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention overcomes the shortcomings of the prior arttechniques by addressing the important manufacturability and yieldissues associated with poor control of the stoichiometry or compositionof Group IBIIIAVIA semiconductor layers. The technique is generallyapplicable to all techniques used to grow Group IBIIIAVIA semiconductorlayers. It is specifically well suited to control the composition oflayers grown using a two-stage technique.

FIG. 1B shows the steps of carrying out the process of the presentinvention. The process comprises at least three steps. In the first stepan initial film comprising at least one of a Group IB, a Group IIIA, anda Group VIA material is deposited on at least a portion of a substrateat a deposition station. In the second step the composition and/orthickness of the initial film on the portion of the substrate ismeasured in a metrology station. In the third step, based on theinformation from the second step, an additional film is deposited on theportion at a tuning station to bring the composition and/or thickness ofthe total film to a target value, wherein the total film is defined asthe sum of the initial film and the additional film. These three stepsmay then be repeated for better accuracy in reaching the target value.The deposition processes carried out at the deposition station(s) and atthe tuning station(s) may be selected from a variety of techniques suchas evaporation, sputtering, electroplating, electroless plating, inkdeposition, melt deposition etc. These techniques may be mixed also. Forexample, an evaporation technique may be used in the deposition stationand an electroplating technique may be used in the tuning station.Furthermore, there may be more than one deposition station and more thanone tuning station to first deposit the initial layer and then to bringthe composition/thickness of the total film to the target value. Variousmeasurement methods may be used at the metrology station(s). Thesemethods include, but are not limited to X-ray fluorescence, X-raydiffraction, X-ray reflectance, profile thickness measurement, opticalreflectance, ellipsometry, 4-point probe measurement etc.

The deposition station(s), metrology station(s) and tuning station(s)may be placed in a system in various configurations. For example, FIG.2A shows a cluster-tool 20 configuration including a deposition station21, a metrology station 22, a tuning station 23, a load-unload station24 and a handler 25 to transport the substrates between variousstations. It should be noted that one or more deposition stations, oneor more metrology stations, one or more tuning stations, one or moreload-unload stations, as well as one or more other stations such ascleaning and/or drying stations, heating stations, reaction stationsetc. may also be added to the cluster-tool 20 of FIG. 2.

The deposition station(s) 21, metrology station(s) 22, tuning station(s)23 and load-unload station(s) 24 may also be placed in-line manner asshown in FIG. 2B and the handler 25 move linearly to move the substratesbetween various stations.

In one embodiment the deposition station and the tuning station are twodifferent stations. In other words, after an initial film is depositedon a substrate at a deposition station and then measured at a metrologystation, an additional film is deposited over the initial film at atuning station that is different from the deposition station.Alternately, the deposition station and the tuning station may be thesame station. For example, an initial film may be deposited on asubstrate at a deposition station. The substrate may then be moved to ametrology station for measurement of the initial film. Then thesubstrate may be moved back to the same deposition station to deposit anadditional film over the initial film and to achieve the target valuesof thickness and/or composition. In this case the deposition stationacts as a tuning station when the substrate is moved into it for thesecond time. As stated before, it is possible that substrate goesthrough more than one deposition stations before arriving at themetrology station. It may then be moved to more than one tuning stationsto achieve the target composition and/or thickness of the depositedtotal film. Metrology stations may also be more than one, each measuringa different important parameter, for example one measuring compositionand another one measuring thickness.

A specific configuration where the deposition station and the tuningstation are the same is schematically shown in FIG. 3. The configurationin FIG. 3 comprises a deposition station 30 and a metrology station 31.A substrate 32 is first placed in the deposition station 30 and aninitial film is deposited onto the substrate 32 as schematicallyindicated by arrows 33. Then, as shown by the dotted lines, thesubstrate 32 is moved into the metrology station 31 and measurements arecarried out on the initial film using a metrology tool 34. The substrate32 is then moved back into the deposition station 30 to further depositan additional film so that a target total film thickness and/orcomposition is reached.

In another embodiment, the present invention may be used in controllingthe composition of a precursor layer employed in a two-stage methodwhere during the first stage of the process sub-layers comprising atleast one of a Group IB, a Group IIIA and a Group VIA material aredeposited on a substrate to form the precursor layer, and during thesecond stage a reaction is carried out to convert the precursor layerinto a Group IBIIIAVIA compound layer. In this case the sub-layerdeposition process is divided into at least two deposition steps and ameasurement step is provided between the at least two deposition steps.The first deposition step deposits an initial sub-layer. If thedeposited sub-layer comprises only one element, the measurement stepmeasures the thickness of the initial sub-layer. If the depositedsub-layer comprises more than one element, such as an alloy, themeasurement step may measure the effective thickness of each element orit may measure the composition of the sub-layer. The subsequentdeposition step(s) after the measurement step then are used to bring thetotal thickness or the composition of the sub-layer to a targeted value.The invention will now be described using various examples.

EXAMPLE 1

A Cu(In,Ga)Se₂ compound layer with a Cu/(In+Ga) ratio of 0.95 andGa/(Ga+In) ratio of 0.3 may be grown using the present invention byfirst depositing a Cu/In/Ga precursor stack on a substrate and thenreacting the stack with selenium, i.e. by selenizing the stack. It isstraight forward to calculate that if the thickness of the Cu sub-layerin the stack is 200 nm, an In sub-layer thickness of 325 nm and a Gasub-layer thickness of 104 nm would be needed to provide the targetcomposition defined above. In processing such a precursor stack, thefollowing steps may be carried out: i) a nominally 200 nm thick Cusub-layer may be first deposited on at least one portion of a substrateat a first deposition station using optimized conditions and in-situthickness control means, ii) deposited Cu sub-layer thickness on theportion of the substrate may be determined at a metrology or measurementstation, iii) if the thickness measured is within an acceptable range(such as within +/−5%) of the desired 200 nm, the portion may be movedto another process station to continue with In deposition, iv) if thethickness measured is outside the desired range, the portion may bemoved to tuning station to adjust the thickness to the desired range, v)the above steps may be repeated for In deposition and Ga deposition.

It should be noted that the approach of the present invention may beused for the deposition of all the sub-layers or only for sub-layers forwhich deposition thickness control is difficult, unstable ornon-optimized. For example, the approach of the present invention may beused for the deposition of the Cu and Ga sub-layers and may not be usedfor the deposition of the In sub-layer if the deposition method for theIn sub-layer offers already a good thickness control.

EXAMPLE 2

Let us now take, as an example, an electroplating technique used tomanufacture Cu/In/Ga precursor stacks on a large number of substrates,such as hundreds or thousands of substrates which may each be 1 ft×1 ftor 1 ft×4 ft in size. Starting with a fresh Cu plating solution orelectrolyte, it may be possible to deposit accurately the 200 nm thickCu layer on the substrates early in the process by applying apre-determined current density for a pre-determined time. As more andmore substrates are electroplated, however, the electrolyte may start toage and the plating efficiency of Cu may drop from its initial value,which may be in the range of 70-100% depending on the chemistry used.Therefore, although the early substrates receive nominally 200 nm thickCu, the Cu thickness on later substrates may start to decrease. If thisreduction of Cu thickness goes un-detected and if the In and Gathicknesses are accurate, the Cu/(In+Ga) molar ratio in the precursorstack would be lower than the targeted value which, for example may bearound 0.95. This would lower the yields once the precursors areconverted into Cu(In,Ga)Se₂ compound layers and solar cells arefabricated on the compound layers. To avoid this problem, the method ofthe present invention measures at a metrology station the thickness ofthe deposited Cu layer, and if the thickness is reduced from the targetvalue, sends the substrate to a tuning station to electroplate more Cu.As an example, let us assume that the Cu thickness for substrate #100 isreduced down to 160 nm although under the same plating conditionssubstrate #1 had a plated Cu thickness of 200 nm. When the Cu thicknessis measured to be 160 nm for substrate #100, this substrate is forwardedto a tuning station, which may be another Cu plating station, andadditional 40 nm of Cu plating is carried out. It should be noted thatfor better accuracy more metrology steps and tuning steps may be addedto the process. In any case, use of the tuning station(s) providesexcellent composition control of the precursor stack in a productionenvironment where a large number of substrates are processed in acontinuous manner. Similar approaches may be used for the deposition ofthe other components of the stack such as the In and/or Ga sub-layers.Metrology may be carried out on every substrate or at certain intervals,for example for every 10th substrate. Computer systems may be used tocollect the metrology data and trends of thickness behavior may beestablished. This data may then be used to predict thickness as afunction of the bath age. It can also be advantageous to use differentbaths for deposition and tuning, which deposition and tuning may useelectroplating if material is being added.

EXAMPLE 3

In a two-stage process employing an electrodeposited Cu—Ga/In precursorstack, the process may be carried out as follows: A Cu—Ga alloy may befirst deposited on a substrate in the form of an initial film at adeposition station. The substrate may then be moved to a metrologystation to determine the molar Cu and Ga content in this initial film.If more Cu is needed to reach the target composition, the substrate maybe forwarded to a tuning station that deposits more Cu on the initialfilm. If more Ga is needed to reach the target composition, thesubstrate may be forwarded to a tuning station that deposits more Ga onthe initial film. Once the Cu and Ga contents are brought to a targetedrange an In sub-layer may be deposited to form the precursor stack.Indium sub-layer deposition may also be divided into at least two steps.In the first step an In initial film may be deposited over the Cu and Gacontaining sub-layer. At a metrology station In content may be measured.Then at a tuning station In content may be brought to a targeted range.This way Cu, In and Ga content of the precursor stack may be brought tothe pre-determined desired range.

EXAMPLE 4

The method of the present invention may be used in a cluster toolapproach where single substrates, such as pre-cut glass substrates, arecoated with the precursor layers. Alternately, the invention may also beused in an in-line processing approach such as a roll-to-roll processingtechnique.

A Cu(In,Ga)Se₂ compound layer with a Cu/(In+Ga) ratio of 0.95 andGa/(Ga+In) ratio of 0.3 may be grown using the present invention byfirst electrodepositing a Cu/In/Ga precursor stack on a flexible foilsubstrate and then reacting the stack with selenium, i.e. by selenizingthe stack. It is straight forward to calculate that if the thickness ofthe Cu sub-layer in the stack is 200 nm, an In sub-layer thickness of325 nm and a Ga sub-layer thickness of 104 nm would be needed to providethe target composition defined above. In processing such a stack, thefoil substrate may be in the form of a roll. The Cu, In, and Ga layersmay be deposited on the substrate in a roll-to-roll fashion. Alldepositions may be carried out in a single machine with Cu, In and Gaelectroplating stations. They may alternately be carried out in twodifferent machines, one tool electroplating two of the elements (such asCu and In, Cu and Ga, or In and Ga) and the other tool depositing thethird element. Alternately Cu deposition, In deposition and Gadeposition may be carried out in three different roll-to-roll platingmachines or tools.

FIG. 4 schematically shows an in-line, roll-to-roll Cu electroplatingsystem comprising a deposition station 40, a metrology station 41 and atuning station 42. As the flexible foil substrate 43 is moved from thesupply spool 44 to the receiving spool 45, it gets plated by Cu. Themovement of the foil substrate may be continuous or it may be step-wiseto plate the substrate with Cu one section at a time. At any time duringprocessing, the deposition station 40 may be activated to carry outelectrodeposition of an initial Cu film on the portion of the foilsubstrate 43 that is within the deposition station 40. The tuningstation 41 may not be ordinarily activated, i.e. it does not deposit Cu,unless it gets a signal from the metrology station 41. If and whenmetrology station senses a change in the Cu target thickness at aportion of the foil substrate 43, a signal is sent to the tuning stationthrough a computer which determines the amount of additional Cudeposition to be carried out on the portion at the tuning station 41.Since the movement of the foil substrate 43 from left to right iscontrolled, the position of the portion at any time is also known.Therefore, when the portion moves into the tuning station 42 the platingcell is activated through application of a voltage between the foilsubstrate and an anode and the pre-determined additional Cu amount maybe deposited on the portion. Subsequent to this the depositionconditions at the deposition station 40 and tuning station 42 may bekept constant until a new measurement is carried out at the metrologystation 41 and the deposition condition at the tuning station isadjusted again to meet the target final Cu thickness on the substrate.

It should be noted that, in the embodiment discussed above, thicknessmeasurement results from the metrology station are used to adjust thedeposition conditions, such as the deposition current and/or voltage, atthe tuning station, keeping the conditions constant at the depositionstation. It is also possible that results from the metrology station maybe interpreted by a computer to adjust the deposition conditions at thedeposition station or both at the deposition station and the tuningstation. For example, if the thickness of the initial Cu film depositedat the deposition station 40, as detected by the measurement at themetrology station 41, is low, then the deposition conditions at thedeposition station 40 may be adjusted e.g. the deposition currentdensity may be increased, so that future portions of the substrate 43 tobe plated receive a Cu thickness that is closer to the target value. Itis also possible to remove material, rather than deposit material, atthe tuning station. For example, in wet processing such aselectroplating, the tuning station may have an electrode in a solutionand means to apply a voltage between the electrode and the surface ofthe substrate when the surface of the substrate is wetted by thesolution. For deposition, a negative or cathodic voltage is applied tothe substrate surface with respect to the electrode. This way, Cu, In orGa may be deposited or plated on the substrate surface if the solutioncontain Cu, In or Ga species. If, on the other hand an anodic orpositive voltage is applied to the substrate surface with respect to theelectrode already deposited material may be anodically dissolved fromthe substrate surface. For example, if the deposition station 40 in FIG.4 deposits Cu on the substrate 43, and if the metrology station 41detects that the thickness of the deposited Cu is more than a targetthickness, then when that portion of the substrate enters the tuningstation 42, an anodic removal step may be carried out to bring thethickness to the targeted value. Cu removal rate and amount may becontrolled by the current density applied between the substrate surfaceand the electrode in the tuning station.

The processes described for Cu deposition may also be carried out forthe In and Ga depositions and a precursor stack comprising Cu, Ga and Inmay be produced with well defined stoichiometry or composition.

Once the precursor layers of the present invention are depositedreaction of the precursors with Group VIA materials may be achievedvarious ways. In one embodiment the precursor layer is exposed to GroupVIA vapors at elevated temperatures. These techniques are well known inthe field and they involve heating the precursor layer to a temperaturerange of 350-600° C. in the presence of at least one of Se vapors, Svapors, and Te vapors provided by sources such as solid Se, solid S,solid Te, H₂Se gas, H₂S gas etc. for periods ranging from 5 minutes to 1hour. In another embodiment a layer or multi layers of Group VIAmaterials are deposited on the precursor layer and the stacked layersare then heated up in a furnace or in a rapid thermal annealing furnaceand like. Group VIA materials may be evaporated on, sputtered on orplated on the precursor layer. Alternately inks comprising Group VIAnano particles may be prepared and these inks may be deposited on theprecursor layers to form a Group VIA material layer comprising Group VIAnano particles. Dipping, spraying, doctor-blading or ink writingtechniques may be employed to deposit such layers. Reaction may becarried out at elevated temperatures for times ranging from 1 minute to30 minutes depending upon the temperature. As a result of reaction, theGroup IBIIIAVIA compound is formed from the precursor.

Solar cells may be fabricated on the compound layers of the presentinvention using materials and methods well known in the field. Forexample a thin (<0.1 microns) CdS layer may be deposited on the surfaceof the compound layer using the chemical dip method. A transparentwindow of ZnO may be deposited over the CdS layer using MOCVD orsputtering techniques. A metallic finger pattern is optionally depositedover the ZnO to complete the solar cell.

Although the present invention is described with respect to certainpreferred embodiments, modifications thereto will be apparent to thoseskilled in the art.

1. A method of forming a multi-element layer that has a predeterminedmolar ratio range of elements therein, the method comprising the stepsof: depositing a plurality of films on a substrate in at least onedeposition station, wherein the deposited plurality of films includeseach of the elements for which the predetermined molar ratio range isdesired; detecting an amount of each of the elements within thedeposited plurality of films in at least one metrology station; andafter the step of detecting, tuning the plurality of films based on theamount of each of the elements deposited to obtain the multi-elementlayer that has the predetermined molar ratio range.
 2. The methodaccording to claim 1 wherein the step of tuning deposits another filmthat includes one of the elements used for the predetermined molar ratiorange.
 3. The method according to claim 2 wherein the step of tuningdeposits the another film using the at least one deposition station. 4.The method according to claim 1 wherein the step of tuning removes aportion of a top one of the plurality of films.
 5. The method accordingto claim 4 wherein the step of tuning removes the portion using the atleast one deposition station.
 6. The method according to claim 1 whereinthe step of detecting is performed using X-ray fluorescence.
 7. Themethod according to claim 1 wherein the plurality of films includes atleast one of Cu, In and Ga.
 8. The method according to claim 7 whereinthe step of tuning deposits another film that includes one of theelements used for the predetermined molar ratio range.
 9. The methodaccording to claim 7 wherein the step of tuning removes a portion of atop one of the plurality of films.
 10. The method according to claim 1wherein the step of detecting occurs a plurality of times, respectivelyafter each of the plurality of films has been applied, and furtherincluding a step of obtaining an actual molar ratio range frommeasurements obtained in each of the plurality of detecting steps. 11.The method according to claim 1 wherein the step of depositing useselectroplating in a bath, and the step of tuning includes the step ofelectroplating out of another bath that is different than the bath. 12.The method according to claim 11 wherein the steps of depositing,detecting and tuning are carried out on a flexible substrate in aroll-to-roll manner.
 13. A method of forming a layer that issubstantially formed of at least one of Cu, In and Ga having apre-determined thickness, the method comprising the steps of: depositingone of a Cu sub-layer, a In sub-layer and a Ga sub-layer on a substratein at least one deposition station; detecting the thickness of thedeposited sub-layer in a metrology station thus obtaining a measuredthickness; and after the step of detecting, tuning the thickness of thesub-layer based upon the measured thickness to provide a tuned thicknessthat is substantially the same as the pre-determined thickness.
 14. Themethod according to claim 13 wherein: the step of depositing deposits aslayers over the deposited sub-layer the remaining other two of the Cusub-layer, the In sub-layer, and the Ga sub-layer; the step of detectingdetects the thicknesses of each of the sub-layer and the layers asmeasured thicknesses; and the step of tuning tunes the thickness of atleast one of the sub-layer and layers based upon the measuredthicknesses to provide tuned thicknesses that are substantially the sameas pre-determined thicknesses for each of the sub-layer and the layers.15. The method according to claim 14 wherein the step of depositing thelayers occurs after the step of detecting the thickness of thesub-layer.
 16. The method according to claim 14 wherein the step ofdepositing the layers occurs before the step of detecting the thicknessof the sub-layer and the layers.
 17. The method according to claim 14wherein the step of tuning removes a portion of at least one of thesub-layer and the layers deposited in the at least one depositionstation.
 18. The method according to claim 14 wherein the step of tuningdeposits at least one other film using the at least one depositionstation to provide tuned thicknesses that are substantially the same asthe pre-determined thicknesses for each of the sub-layer and the layers.19. The method according to claim 13 wherein the step of detecting isperformed using X-ray fluorescence.
 20. The method according to claim 13wherein the step of depositing uses electroplating.
 21. The methodaccording to claim 20 wherein the step of tuning uses electroplating.22. The method according to claim 21 wherein the steps of depositing,detecting and tuning are carried out on a flexible substrate in aroll-to-roll manner.
 23. The method according to claim 13 wherein thestep of tuning uses anodic dissolution.
 24. The method according toclaim 23 wherein the substrate is a flexible substrate and the steps ofdepositing, detecting and tuning are carried out on the flexiblesubstrate in a roll-to-roll manner.